in Vol. 5 - January Issue - Year 2004
Drum Blasting Machines for the Blasting of Bulk Goods
Overall view of a drum blasting machine with loading device
View of the drum through open loading door
Loading of the drum blasting machine
Batch distribution after the blasting process (during the start-up phase)
Founded in 2001, the focus of AGTOS is on the manufacture of special purpose machinery for the surface preparation sector. Special emphasis is put on the design and manufacture of turbine wheel shotblast units, filter systems, and conveyor and transport technology. A second field of activity is the service sector, which includes the delivery by the manufacturer of expendable and spare parts for its own machines and the machines of other manufacturers, as well as maintenance and repair work. In May 2003, the company moved into a new production facility and office building located in Emsdetten. The location is also home to a branch office of SLF, a partner company to AGTOS that manufactures coating plants and equipment.
These ultra-modern blasting machines are based on an old, time-tested principle. Work pieces have been processed for decades in drum-type blasting machines and for the most part, the design of these machines is just as old. New drum blasting machines offer a wealth of convincing new technical details in a cost-effective design that offers many operating advantages. What exactly are the strengths of these machines? What advantages do they have over other types of blasting machines?
A global player in the automotive supply sector is focused on manufacturing new clutch plates and reworking used ones. Here, the company was faced with two challenges with respect to blasting technology: Previously, it had outsourced the hardening of some of its clutch springs. But the rejection rate for these pieces was extremely high. For this reason, a decision was made to process all components in-house.
As part of a plan to expand its production of link plates and link pins, which are manufactured in a different production building, the company set the following goal: Double throughput and automate the entire blasting process. It approached AGTOS of 48282 Emsdetten in Germany with these specifications. In both cases, the goal of blasting is to harden the work piece surfaces. Ralf Scheibler, a Sales Engineer at AGTOS, first met with the customer and Ljubica Petrovic, Chief Designer, to discuss several solutions to the problem.
The principle of the tumble belt is widely used in the processing of bulk materials. A rubber belt forms a trough-like depression and work pieces collect in the trough, where they are kept in motion during the blasting process. In this way, the work pieces are processed on all sides. However, this type of machine is not advantageous for every work piece. The transition points from the moveable belt to non-moving machine parts can cause complications with certain kinds of work piece shapes. Parts may become trapped, fail to discharge or suffer damage. This can lead to costly interruptions in the production process. Repairs are the result.
Drum blasting machines are unaffected by these contingencies. The drum is constructed as a single piece and is made of perforated manganese steel to increase its resistance to wear and abrasion. The size of the perforated holes depends on the dimensions of the work pieces as well as the grain-size and amount of abrasive run-off. Cleverly engineered structural devices promote the stirring of the work pieces.
A second type of blasting machine for processing bulk material is the continuous-flow blasting machine. In machines of this type, the work pieces are blasted as they pass through a pipe-shaped machine frame. However, the behavior of work pieces as they pass through the machine varies widely. This limits the flexibility of these machines. Because processing takes place in a long, horizontal machine frame, which requires the addition of appropriate feed and take-off devices, the space required by these machines is yet another disadvantage.
With respect to bulk materials that are processed in batches, it is often uneconomical to process these materials in a continuous flow machine and then to reprocess them afterwards in batches. In cases like these, the drum blasting machine is often a better fit for the existing operating conditions. In plants where continuous operation is the rule, the installation of two smaller drum blasting machines, operating in tandem mode, is recommended. This makes it possible to achieve a “quasi continuous process”. Process safety also increases because a second machine is always available in the event that one of the machines fails.
After consideration of all the criteria, a decision was made to build drum blasting machines of different sizes for both applications. The customer places special importance on the process safety of the machines and the reproducibility of blasting results. Process safety is an inevitable factor with regard to the continuity of quality in the final product. In this respect, modern drum blasting machines have characteristics not exhibited by conventional machines. Electronic monitoring and control of process parameters make it possible for the operator to access information about the blasting process at any time. This can apply, for example, to information about blast duration, the speed of the turbine and drum, or the throughput rate of the abrasive. In addition, fault reports are displayed in detail in order to ensure their quick rectification. The machine is placed in fault mode and the blasting process is interrupted as soon as a previously programmed step in the work cycle fails to take place. The source of the problem is displayed and can be quickly eliminated. An important criterion for the customer was that no work pieces remain inside the machine upon completion of the blasting process. This is excluded in the case of drum blasting machines.
Process safety also includes the thorough mixing of the batch being processed and the associated, uniform surface quality of all work pieces. This is achieved through the further development of the appropriate mechanical devices.
The work pieces are located in a standard container. This container is placed in a charging device or emptied directly. The charging device is raised into position in front of the machine door, which immediately opens. After arriving at the correct height, the charging device pivots and empties the work pieces into the drum. Matched to the sensitivity of the work pieces, this process can take place very gently. The control parameters can be set manually or by SPS. This prevents damage to the work pieces. At this time, the drum is located in the loading position. Before the blasting process begins, the door of the machine closes and the drum pivots to take up an ideal position in front of the high-performance turbine. At the same time, the drum rotates around its own axis.
The blasting process begins and continues until all of the work pieces have been thoroughly mixed and blasted on all sides. The blasting chamber is lined with materials that exhibit a high degree of stability with respect to the abrasive. The primary lining in the direct blasting area of the high performance turbine consists of high-strength, exchangeable steel plates. During the blasting process, the door remains tightly closed and it can only be re-opened when the blasting chamber is free of dust. This takes approximately 10-15 seconds. The dust created by the blasting process is extracted during blasting by a slight suction force and is then removed by the machine’s filter unit.
After the blasting process, the drum pivots into the unloading position. Here, too, the number of work pieces to be unloaded and the intensity of the unloading process can be defined and controlled. The work pieces move onto a screening conveyor, which separates the work pieces from the rest of the abrasive and then transports them to waiting collection containers.
The mobility of the drum is a unique characteristic of the machines. In comparison to older machinery, it makes it possible to avoid damages to the work pieces. The drum’s mobility also guarantees that the drum is emptied completely, even when working with small parts that stick or jam. An additional advantage is that work pieces can be processed and afterwards emptied in controlled amounts into containers of different sizes, in the event this is necessary for subsequent processing steps.
Turbines and abrasive processing loop
The high-performance turbines propel the abrasive onto the work pieces. Depending on the blasting goal, speeds of more than 100 m/sec are achieved. The high-performance turbine of the German manufacturer is designed as a single-disk turbine. For this reason, the turbine is low in wear and maintenance friendly. The turbine blades are inserted into a carrier plate, which is protected against wear and tear by a wear plate. Compared to the double-disk turbines widely available in the marketplace, it was possible to increase the performance of the AGTOS turbine by approx. 20%. Fewer expendable parts are used and maintenance is both faster and simpler. Fair prices for spare parts also lead to additional savings. Unlike most widely used expendable parts, which are made by a casting process, the company makes its expendable parts from tool-grade steel, which increases the service life of the components.
Maintenance- and environmentally-friendly cartridge filter system
The filter technology also sets new standards and makes a significant contribution to process safety. Dust-laden air is extracted from the blasting chamber and flows through the impact separator, where the initial separation of heavy dust particles occurs. After this, dust enters the lower filter chamber. Dust is removed as the air flows through the filter cartridges and clean air then flows upward into the clean gas chamber.
The filter cartridges are cleaned in programmable intervals by pulses of compressed air. The dust particles are fed to a hopper located beneath the filter, where they are collected in a dust collection container made of metal or paper. Clean air is released to the outside environment through openings in the side of the upper filter housing or is returned back to the production hall. Should a filter cartridge be destroyed, additional filter inserts collect the dust to the greatest extent possible.
The filter cartridges have a conical shape because this permits the compressed air impulse to more directly impact the inside of the side walls and the attached coating of dust. In comparison to other cartridge types, the overall area of the filter surface is smaller. Despite this, the special structure of the filter medium makes it possible to draw more air per m2 through the cartridge. In addition, the use of a rougher fold provides for better dust adhesion and cleaning because the dust is dislodged more easily by the compressed air impulse.
In comparison to the cylindrical sidewalls of conventional filter cartridges, the conical shape of the filter cartridges alone is enough to achieve a better utilization of the compressed air pulses. The disposal process illustrates yet another advantage: the loading-bearing metal body of the filter cartridge is reusable. Only the filter medium requires disposal. Depending on the type of dust collected, it can usually be burned or disposed of as common industrial waste. This eliminates additional disposal costs.
Newly developed slide-in inserts simplify the exchange and replacement of cartridges. The cartridges are pulled from the filter unit and can then be replaced outside the filter in a clean environment. The cartridges are still precisely positioned when reinserted and always install with the proper contact pressure.
The company manufactures its blasting machines and filters using screw-based construction techniques. Compared to welded designs, this method demands a more precise type of construction. The advantages become clear during transport and assembly of the components. In addition, this often results in improved rigidity and a more robust machine design.
Working in close cooperation with its client, the German manufacturer developed the modern drum blasting machine in two drum sizes. In doing so, priority was given to the demands of the client on the blasting and transport technology. As is usually the case when developing special machine designs, modifications were made to the original plans. The excellent cooperation between the client’s project managers and the sales, construction and manufacturing departments at AGTOS made these successful solutions possible.